1. Field of the Invention
The invention relates to an electric power steering system.
2. Description of the Related Art
There is an electric power steering system (EPS) that uses a motor as a driving source. The electric power steering system includes a driving circuit that supplies a driving current to the motor on the basis of a power supply voltage, a control device that controls the operation of the driving circuit and a relay circuit that turns on or off a relay contact provided in a first power supply path that connects the driving circuit and a power supply to each other.
Normally, a capacitor is provided between the relay contact and the driving circuit in order to stabilize the supply of power from the power source. However, in a state where such a capacitor is provided, when the charge amount of the capacitor is small at the time when the relay contact enters an on state, an inrush current may flow from the power supply to the relay contact.
Then, for example, as described in Japanese Patent Application Publication No. 2007-276706 (JP-A-2007-276706), the EPS includes a precharge circuit that is interposed in a second power supply path that bypasses the relay contact in order to prevent occurrence of the inrush current. Then, an ignition switch is turned on, the capacitor is charged (precharged) by the precharge circuit, and then the relay contact is turned on.
In such an EPS, stuck-open fault, in which the relay contact remains turned off, may occur, for example, when foreign matter is caught in the relay contact. Then, there is known an EPS that includes the precharge circuit provided with a switching element and a discharge circuit. The switching element is used for switching between the execution and termination of precharge. The discharge circuit has a circuit configuration similar to that of the precharge circuit, and is provided between the capacitor and the driving circuit. In the EPS, after the relay circuit is activated in order to turn on the relay contact, the precharge is stopped, and a voltage difference between both terminals of the relay contact is detected in a state where the capacitor is discharged to thereby determine whether the relay contact has a stuck-open fault.
When the relay contact is normal and has no stuck-open fault, the relay circuit is activated and then the relay contact is actually turned on. Therefore, even when the capacitor is discharged in a state where precharge is stopped, the capacitor-side terminal voltage of the relay contact does not decrease but is kept at the power supply voltage. On the other hand, when the relay contact has a stuck-open fault, the relay contact is not actually turned on even when the relay circuit is activated. Therefore, when the capacitor is discharged in a state where precharge is stopped, the capacitor-side voltage of the relay contact decreases. Thus, when there is a voltage difference between both terminals of the relay contact after the capacitor is discharged in a state where precharge is stopped, it is determined that the relay contact has a stuck-open fault.
However, in the existing configuration, in order to detect the above-described stuck-open fault, the precharge circuit need to have a function of switching between the execution and termination of the precharge. Furthermore, it is necessary to provide a discharge circuit having a scale similar to that of the above-described precharge circuit between the capacitor and the driving circuit. As a result, the number of components of the control device increases.